1. Field of the Invention
This invention relates to radiation monitors and more particularly, to measuring instruments suitable for monitoring radiation of electric fields within a frequency range extending to the vicinity of 100 GHz.
2. Description of the Prior Art
The essential components of instruments capable of monitoring radiation over a range of frequencies, are the electrical field sensitive elements. Their characteristics, structure, and interconnection are selected to achieve a sensitivity to the field that is relatively flat over the frequency ranges of interest. Thermocouples and semiconductor diodes are common sensing elements. It has been recognized that each type of element, while having inherent advantages and limitations, also exhibits frequency limitations. Thus for example, diode sensors have been shown to operate most effectively at lower frequencies, e.g. below 1.5 GHz, while thermocouple sensors operate most effectively at higher frequencies, e.g. above 300 MHz.
The characteristics of various sensors also have shown that diodes are most effective when applied in conjunction with the high reactance of electrically short dipole antennas. In contrast, the best sensitivity of thermocouple elements is achieved when low resistivity materials are employed in a thin-film configuration in conjunction with low reactance dipoles. There are other recognized characteristics which lead to corresponding limitations. At higher frequencies the reactance of diode sensors may be dominated by shunt capacitance effects which severely limit their high frequency response. At the other extreme, thermocouple sensors are limited in their application to the low frequency region due to difficulties in developing thermocouples of sufficiently high film resistivity to operate in conjunction with the higher reactance dipoles and with sufficient sensitivity to provide adequate signal levels.
One form of effective field measuring equipment is shown in the inventor's U.S. Pat. Nos. 3,641,439 and 3,794,914, issued on Feb. 9, 1972 and Feb. 26, 1974, respectively. These patents disclose a near-field radiation monitor utilizing thin-film thermocouples positioned in quadrature to measure relatively high frequency electric fields.
As further described in the inventor's article Broad-Band Isotropic Electromagnetic Radiation Monitor, published in the November 1972 I.E.E.E. Transactions on Instrumentation and Measurement, one may utilize sensors of the general type described mounted along three mutually orthogonal axes in order to achieve isotropic performance. On the other hand, to date, no suitable equipment is known to be available that is capable of operating over the broad frequency range contemplated herein.